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Figure 67 Transmission of multicast protocol packets
Source
Site 1
S: 192.1.1.1/24
G: 225.1.1.1
Share-Group: 239.1.1.1
The work process of multicast protocol packets is as follows:
1.
Receiver sends an IGMP membership report for multicast group G to CE 2. CE 2 creates a local
state entry (*, 225.1.1.1) and sends a join message to the VPN RP (CE 1).
2.
After receiving the join message from CE 2, the VPN instance on PE 2 creates a state entry (*,
225.1.1.1) with the upstream interface being the MTI. Then, PE 2 processes the join message.
Now, the VPN instance on PE 2 considers that the join message has been sent out of the MTI.
3.
PE 2 encapsulates the join message by GRE. Its BGP interface address is the multicast source
address and the share-group address is the multicast group address, converting it into a normal,
public network multicast data packet (11.1.2.1, 239.1.1.1). PE 2 then forwards it to the public
network.
4.
The multicast data packet (11.1.2.1, 239.1.1.1) is forwarded to the public network on all the PE
devices along the share-MDT. After receiving this packet, every PE device de-encapsulates it to turn
it back into a join message to be sent to the VPN RP. Then, each PE device examines the join
message. If any PE device finds that the VPN RP is in the site it interfaces with, it passes the join
message to the VPN instance on it. Otherwise, it discards the join message.
5.
When receiving the join message, the VPN instance on PE 1 considers that it received the message
from the MTI. PE 1 creates a local state entry (*, 225.1.1.1), with the downstream interface being
the MTI and the upstream interface being the one that leads to CE 1. At the same time, it sends a
join message to CE 1, which is the VPN RP.
6.
After receiving the join message from the VPN instance on PE 1, CE 1 creates a local state entry
(*, 225.1.1.1) or updates the entry if it already exists. By now, the construction of an RPT across
the public network is completed.
For more information about GRE, see Layer 3—IP Services Configuration Guide.
Delivery of multicast data packets
After the share-MDT is established, the multicast source forwards the VPN multicast data to the receivers
in each site along the distribution tree. The VPN multicast packets are encapsulated into public network
multicast packets on the local PE device, transmitted along the share-MDT, and then de-encapsulated on
RP
CE 1
PE 1
BGP: 11.1.1.1/24
BGP: 11.1.3.1/24
PE 3
P
PE 2
MD
BGP: 11.1.2.1/24
Public instance BGP peers
VPN instance join (*, 225.1.1.1)
Public instance join (11.1.2.1, 239.1.1.1)
232
Receiver
CE 2
Site 2
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